.middle {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.menu {
    overflow: hidden;
}

.item {
    border-top: 1px solid #2980b9;
    overflow: hidden;
}

.btn {
    position: relative;
    display: block;
    padding: 8px 10px;
    background: #dcecfa;
    color: #666;
}

.btn::before {
    content: "";
    position: absolute;
    width: 14px;
    height: 14px;
    left: 20px;
    bottom: -7px;
    transform: rotate(45deg);
}

.btn i {
    margin-right: 10px;
    text-align: left;
}

.smenu {
    background: rgb(248, 248, 248);
    overflow: hidden;
    transition: max-height .3s;
    max-height: 0;
}

.smenu a {
    display: block;
    padding: 8px 13px;
    color: #000;
    font-size: 14px;
    margin: 4px 0;
    position: relative;
}

.smenu a::before {
    content: "";
    position: absolute;
    width: 6px;
    height: 100%;
    background: #3498db;
    left: 0;
    top: 0;
    transition: .3s;
    opacity: 0;
}

.smenu a:hover::before {
    opacity: 1;
}

.item:target .smenu {
    max-height: 100%;
}
